To push the ground down for a basement...you have to use the terrain tool.

Use the one that shows the curve going down and hold the left mouse button down until you have the depth you want in a small area. Next you can level the rest of the ground to match the new level. For that you use the level one that has the arrows above and below on it.

You need to go down far enough so you are able to place a door to go out.

Well...I haven't acually made a basement myself but I did ask my son who has and he told me this.

To make it the right depth...you have to use one panel of wallpaper on the wall to let you know how deep to go.

You have to leave one square of ground next to the wall inside and the rest of the floor will be the depth of the wallpaper.

You also have to put a foundation down first to be able to have a basement.

You can dig out some ground outside next to a wall and then be able to place a door in the wall to go out.

Of course I have to try all of this out myself to see how it all works but everytime I go to do it I get sidetracked with doing something else. Sigh. I will try it soon and see for myself how it works.

I figured it out VHL. I made a foundation and took the middle out then lowered the ground until I could put in a staircase up to the first floor. Now I have a cool new pool in my basement. It's a liile wierd but fun.